In several important lung diseases, including emphysema, vascular remodeling after ARDS, and[unreadable] possibly primary pulmonary hypertension, there is a prominent cytotoxic response of pulmonary[unreadable] microvascular endothelial cells (MV ECs) leading to a diminution in capillary density. While there is no doubt[unreadable] that reactive oxygen species play an important role in this response, the specific target(s) of ROS that serve[unreadable] as a sentinel molecule - triggering cell death when the oxidant stress is so severe as to preclude effective[unreadable] recovery or threaten the organism with mutation - is not known. In this regard, an intriguing target of ROS is[unreadable] mitochondrial (mt) DNA. The mitochondrial genome is at least 30-fold more sensitive to oxidative damage[unreadable] than nuclear DNA, and our work during the initial funding period supports the hypothesis that oxidative[unreadable] mtDNA damage is a proximate trigger for lung EC death. If this hypothesis is valid, then mtDNA repair[unreadable] pathways could emerge as a new target for intervention in oxidant-induced MV EC death and capillary[unreadable] rarefaction. However, there is a stark lack of the information about the details of mtDNA repair in MV ECs[unreadable] and other cells. For example, while it is suspected that the base excision repair mechanism is the dominant[unreadable] pathway defending the mitochondrial genome from oxidative damage, the presence of other DNA repair[unreadable] pathway components suggests that a more complicated repair paradigm could be operative. In addition,[unreadable] neither the identities of the enzymes participating in mitochondrial base excision repair nor the rate limiting[unreadable] determinants are known. Against this background, the Aims of this proposal are to: (1) Identify the dominant[unreadable] pathway repairing oxidative damage to the mitochondrial genome in MV ECs; (2) Determine the rate-limiting[unreadable] functional steps in mtDNA repair; and, (3) Establish the critical operational enzymes repairing mtDNA in MV[unreadable] ECs. Collectively, these studies will provide the first detailed understanding of pathways defending the[unreadable] mitochondrial genome in this important lung cell population and determine the suitability of mtDNA repair[unreadable] enzymes to serve as isolated targets for intervention. Importantly, the outcome of these studies also will set[unreadable] the stage for pre-clinical, translational experiments on the ability of augmented mtDNA repair to suppress[unreadable] capillary rarefaction in relevant animal models.